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   CalcHEP uses  the Breit-Wigner formula  for
propagators of virtual particles with finite 
width. This trick is needed to avoid the integral
divergence. From the other side such modification
of propagators sometimes leads to breaking of 
gauge invariance which is responsible for mutual 
cancellation of contributions of different 
diagrams.
  This menu function displays the next level menu 
which control implementation of Breit-Wigner 
formula. 
